1. Field of the Invention
This invention relates to apparatuses for measuring the crystallization temperature of fluids, particularly fluids comprising brine used in oil well completion, workover and drilling operations.
2. Description of Relevant Art
The need to know the crystallization temperature of fluids such as brines and brine-based fluids used in completion, workover and drilling operations in a subterranean formation, particularly a hydrocarbon bearing subterranean formation, is well known. This need is increasingly important for fluids intended for use in the low temperatures and high pressures commonly experienced at the mud line in deepwater wells where any crystallization is undesirable. See Michael A. Freeman et al., High Pressure Crystallization of Deep-Water Completion Brines, SPE 58729 (2000).
Salt crystals, which form and precipitate at or below the crystallization temperature, can lead to problems. Such problems include: plugging of filtration units; and settling in the tank and consequent altering the density of the fluid pumped, possibly to the point the density is insufficient to control formation pressures. Additional crystals forming in brines may also increase the brine viscosity to the point the brine appears as a frozen solid, resulting in line plugging and pump seizure.
Thus, crystallization temperature of a brine or fluid for use in wellbores penetrating subterranean formations is normally part of the specifications for such fluids. The actual crystallization temperature of a brine is said to be that temperature at which a solid will begin to form out of solution if given sufficient time and proper nucleating conditions. The solid may be either salt or freshwater ice. Salt crystals have a smaller specific volume than brine. Thus, brine will not expand in volume during crystallization as seen when drinking water freezes.
The crystallization temperature of a fluid at a given density can be varied by adjusting the composition and concentration of the salts in the fluid.
Three different crystallization temperatures are often quoted for brines. These three crystallization temperatures are:
FCTA (First Crystal To Appear);
TCT (True Crystallization Temperature); and
LCTD (Last Crystal to Dissolve).
The American Petroleum Institute (API) defines FCTA as: xe2x80x9cThe temperature corresponding to a minimum in a plot of temperature during cooling, or the temperature at which visible crystals start to form.xe2x80x9d FCTA will generally include some xe2x80x9csupercooling effectxe2x80x9d or cooling below the actual crystallization temperature. See API Recommended Practice 13J at ¶7.1.12a.
The API defines TCT as: xe2x80x9cthe maximum temperature reached following the supercooling minimum, or the inflection point in cases with no supercooling,xe2x80x9d in a plot of temperature during cooling cycle. TCT will equal FCTA if there is no supercooling. See API Recommended Practice 13J at ¶7.1.12b.
The API defines LCTD as: xe2x80x9cthe temperature at which crystals disappear, or the inflection point on the heating curve,xe2x80x9d in a heating cycle. See API Recommended Practice 13J at ¶7.1.12c.
The API has warned that the accuracy of a crystallization temperature testing method depends on several factors, most importantly the xe2x80x9csupercooling control.xe2x80x9d According to the API, supercooling or the supercooling effect occurs when a brine is cooled below its actual crystallization temperature. Supercooling may be minimized by slow cooling rates and nucleation of crystallization with selected solid surfaces. Solid surfaces considered effective nucleators for brines include, for example, barium oxide, barium hydroxide, calcium carbonate, and bentonite. Only a very small amount of nucleators is said to be needed to reduce supercooling. See API Recommended Practice 13J at 7.1.14-7.1.15.
According to the API, the best measure of the crystallization temperature of a brine is the TCT. This measured crystallization temperature is said to best represent the temperature at which crystals will precipitate from a brine. FCTA is typically lower than TCT and LCTD is typically higher than TCT. The difference between FCTA and TCT is said to represent the degree of supercooling. If this difference exceeds 5xc2x0 F. (3xc2x0 C.), the API recommends repeat of the measurements for crystallization point at a slower cooling rate. See API Recommended Practice 13J at 7.1.20.
In the oil and gas industry, the most common method of determining crystallization temperature of brine calls for cooling a sample of the brine and observing the decreasing temperature until crystals begin to form. The minimum temperature reached before crystallization is recorded as the FCTA temperature. The maximum temperature obtained immediately after crystallization is recorded as the TCT. The sample is then allowed to warm by discontinuing cooling and is observed until all crystals formed during the cooling cycle have dissolved. The temperature at which all of the crystals have dissolved is recorded as the LCTD temperature. See API Recommended Practice 13J 7.3.
This common procedure does not provide for measurement under high pressure. Measurements under pressure, particularly high pressure, are desired because the increased pressure better simulates the conditions found in a subterranean formation. However, measuring crystallization temperature under high pressure has been viewed as not feasible or difficult, because of the need to have a person directly view or xe2x80x9ceyeballxe2x80x9d the sample for reading the measurements.
A need exists for apparatuses and techniques that afford measurement of crystallization point in fluids under high pressure.
A method and apparatus are disclosed for determining or measuring the crystallization temperature of fluids at high pressure (i.e., pressures exceeding atmospheric pressure and reaching about 5,000 psig to about 10,000 psig or even as high as about 20,000 psig or more, preferably simulating pressures in a wellbore penetrating a subterranean formation.
The apparatus of the invention comprises a test cell and a pressurization vessel for pressurizing the test cell or for holding or enclosing the test cell at pressures greater than atmospheric pressure and preferably at pressures approximating subterranean formation pressures. Preferably the test cell and pressurization vessel are a single vessel but alternatively they could comprise distinct or separable vessels. The apparatus further comprises a thermometer or other measurer of temperature, i.e., a temperature probe, and optical fibers, preferably comprising a fiber optic probe, capable of being inserted in the test cell and in test fluid to be tested in the test cell.
At least one optical fiber is connected to an external light source and at least one optical fiber is connected to an external light detector, through a suitable high pressure seal. These optical fibers afford observation or determination of crystal formation and dissolution in said sample without need for a person to visually watch said sample.
The apparatus further preferably comprises a jacket for receiving and circulating coolant or heat transfer fluid around the test cell to facilitate cooling of the sample for crystallization.
In the method of the invention, a sample of fluid comprising brine is placed in a test cell, preferably in the apparatus of the invention, and put under pressure (greater than atmospheric pressure). Pressurization is preferably obtained with a positive displacement pump which affords information on changes in the volume of the test fluid, as when crystals are formed in the fluid reducing the fluid volume when compared to the same fluid with such crystals dissolved therein. The sample is then cooled until crystals begin to form and the temperature of the sample when such crystals begin to form is recorded. The sample is then allowed to warm and the temperature when all crystals formed during cooling have dissolved is recorded. Additional temperatures during the cooling and heating or warming cycles may be recorded as desired.
The point at which the crystals begin to form or have completely dissolved may be assessed, observed or determined through the optical fibers, transmitting light to the sample and detecting reflection of said light and transmitting same to a light detector. Crystals will reflect light differently than fluid.
Alternatively, or additionally, changes in volume may be detected and determined. Thus for some brines, the point at which the crystals begin to form or have completely dissolved may also be assessed through a change in volume of the fluid. Further, alternatively or additionally, the point at which the crystals begin to form or have completely dissolved may be assessed by temperature pattern, since formation of the crystals will emit heat into the fluid.